1. Field of the Invention
The present invention relates to an image fixing unit for use in a wet-type electrophotographic copying machine.
2. Background of the Related Art
In a wet-type electrophotographic copying machine, there is conventionally known an image fixing method comprising the steps of developing a latent electrostatic image formed on a photoconductive member to a visible toner image with a liquid-type developer, transferring the visible toner image to a transfer sheet, and fixing the transferred image to the transfer sheet by bringing a heat application means into direct contact with the toner-image-bearing surface of the transfer sheet.
As shown in FIG. 1, for example, a heat-application roller 1 with an inner heater 3 serving as a heat-application means incorporated therein is disposed along a transfer sheet transportation path through which an unfixed-toner-image bearing transfer sheet P advances in the direction of the arrow, and a pressure-application roller 2 is in pressure contact with the heat-application roller 1. More specifically, a cam surface of a pressure-application cam 5 is in pressure contact with a shaft portion of the pressure-application roller 2 by the urging force of a spring 6, so that a predetermined pressure is applied to the pressure-application roller 2, thus the pressure-application roller 2 is brought into pressure contact with the heat-application roller 1. Around the heat-application roller 1, a thermistor 7 and a temperature-control fuse 8 are provided to control the temperature of the heat-application roller 1. A transfer sheet separating pawl 10 and a silicone-oil-application felt 11 are situated in contact with the surface of the heat-application roller 1 and prevent the transfer sheet from winding around the heat-application roller 1.
However, the above-mentioned conventional image fixing unit for use in an electrophotographic copying machine has the shortcoming that the image quality of fixed image varies depending on the kind or type of a transfer sheet P employed. In particular, when transfer sheets which scarcely absorb a carrier liquid contained in a liquid developer, such as a transparent film for overhead projectors (OHP), an intermediate original sheet and art paper, are employed, an unfixed toner image on the transfer sheet is caused to corrupt or flow as shown in FIG. 2 in the course of image fixing and accordingly the fixed image becomes illegible. In the figure, the arrow indicates the transporting direction of the transfer sheet during the image fixing process.
In order to solve the above problem, an image fixing test was carried out to analyze the relationship between carrier liquid retention ratio in a developer deposited on a variety of transfer sheets and the occurrence of image flow in the obtained images.
The above-mentioned carrier liquid retention ratio of the developer was obtained by the following formula: ##EQU1##
To obtain the above-mentioned carrier liquid retention ratio of the employed developer, a liquid-type developer was prepared by dispersing and diluting a thermal cross-linking toner in a carrier liquid such as "Isopar H" made by Exxon Co., with a dilution ratio of 50 g/l. Latent electrostatic images were developed to toner images on a commercially available transparent film for OHP (Trademark "TYPE PPC-DX" (A4 size) made by Ricoh Company Ltd.), by use of the above-mentioned developer and a commercially available electrophotographic copying machine. The thus developed OHP film was taken out from the electrophotographic copying machine before reaching an image fixing unit. The weight of the OHP film was measured. After this measurement, the OHP film was then dried through the image fixing unit of the electrophotographic copying machine to evaporate the carrier liquid, and the weight thereof was measured. In the above-mentioned image fixing test, the fixing temperature was adjusted to 140.degree. .+-.10.degree. C. and the paper feeding linear speed set at 350 mm/sec.
The results are given in Table 1.
TABLE 1 ______________________________________ Image Flow Solvent Intermediate Art Plain Retention OHP Original Paper Paper Ratio (%) Film (*) Sheet (**) (***) (T-6200) ______________________________________ 77 1 1 2 5 72 2 2 3 5 68 2 3 3 5 65 2.5 3 3.5 5 60 3 3 4 5 55 4 4 5 5 45 4.5 5 5 5 9 5 5 5 5 ______________________________________ (*) A commercially available transparent film for the OHP "TYPE PPCDX" made by Ricoh Company Ltd. (**) A commercially available intermediate original sheet "TYPE TA" made by Ricoh Company Ltd. (***) A commercially available duplex coated paper.
In Table 1, the image flow was evaluated by visual inspection and the degree of the image flow is expressed by the following ranking scale:
1: Image flow is observed in all the obtained images and characters are illegible. PA1 2: Image flow is slightly observed in the obtained images and the image density of the solid image areas is low. PA1 3: Image flow is less, but the characters in the solid image areas spread. PA1 4: Image flow is hardly observed and the characters in the solid image areas slightly spread. PA1 5: Image flow is not observed at all and the characters in the solid image areas are clear.
The results in Table 1 indicate that when the carrier liquid retention ratio in the developer deposited on the transfer sheet is 65% or more, image flow takes place to some extent in the fixed images on the OHP film, the intermediate original sheet and the art paper.
To prevent the image flow in the course of image fixing, several trials have been made to decrease the carrier liquid retention ratio prior to image fixing by heating. For example, a pair of a rigid blotter roller and an elastic back-up roller is provided upstream of thermal image fixing rollers (heat-application roller and pressure-application roller). The blotter roller absorbs the carrier liquid contained in the developer deposited on the transfer sheet. However, since the above-mentioned blotter roller is a rigid roller and the back-up roller is elastic, the nip between the two rollers is in a concave shape due to the deformation of the elastic back-up roller. Accordingly, the transfer sheet is discharged from the above-mentioned nip between the two rollers, with a front surface thereof curled concavely (referred to as "face-curl"). Thus, such a rigid blotter roller has the shortcomings that transfer sheet transportation reliability cannot be ensured due to the face-curl of the transfer sheet and therefore there is the risk that the transfer sheet is easily wound around the blotter roller.
In order to solve this problem, there is proposed a method of securely guiding the curled transfer sheet along the transfer sheet path. For example, a paper guide plate and a paper separating pawl are arranged around the blotter roller. However, the toner deposited on the transfer sheet has not yet been fixed completely in this stage, so that there is the risk that the toner images formed on the transfer sheet are damaged or smeared by these guide plate and paper separating pawl.
In the case where the distance between a transfer unit and an image fixing unit is relatively long, a pair of transporting rollers are conventionally provided halfway between the transfer unit and the image fixing unit, especially for the transportation of small-sized transfer sheets. However, when a pair of the blotter roller and the back-up roller and a pair of the transporting rollers are arranged halfway between the transfer unit and the image fixing unit, the manufacturing cost is increased and paper transportation reliability is decreased.